Dengue viruses are members of the flavirus group of togaviridae that contain a positive stranded RNA genome of approximately 10-12 kilobases. We employed recombinant DNA techniques to investigate the molecular biology of dengue virus with the intent of developing immunoprophylactic measures against this virus group that is epidemic in many geographical areas. The 42S full-length RNA from dengue virus type 4, produced in C6/36 mosquito cells, was isolated and tailed with poly(A) at the 3'-terminus using E. coli poly(A) polymerase. Complementary DNA was synthesized by reverse-transcription using oligo(dT) as a primer and subsequently converted to double stranded DNA by oligo d(C) tailing and oligo d(G) priming. The dengue DNA product was inserted into the Pst I site of pBR322 using the dG/dC linker sequences. E. coli transformants containing dengue virus specific sequences were identified by in situ colony hybridization. Characterization of recombinant plasmids from several transformants showed that these DNA inserts were short in length ranging from 200 to 300 base pairs. Nevertheless, the dengue specific sequences should prove useful for reverse-transcription extension of virion RNA and relplicative for RNA in order to clone longer, and uyltimately full-length cDNA copies of the dengue genomic RNA sequence. Cloned dengue virus DNA will be used for: (l) mapping and sequencing of viral genes; (2) synthesis of viral polypeptides; and (3) construction of full-length infectious dengue virus DNA.